(1) Field of the Invention
The invention relates to a controllable hydraulic bearing for damping vibrations in a defined frequency band. The bearing includes an elastomeric body, two chambers filled with a magneto-rheological fluid and connected by at least one channel, the two chambers having at least partially flexible chamber walls, wherein the chamber walls have a buckling spring rate representing a measure for a pressure change due to the volume displacement between the chambers, and at least one electromagnet generating a magnetic field in the region of the channel.
(2) Description of Related Art
Controllable hydraulic bearings in different configurations are used in motor vehicles, for example, as hydraulic supports for damping the chassis or the engine, predominantly for damping vibrations in a defined frequency band. Hydraulic bearings or hydraulic bushings include an elastomeric body capable of supporting static preloads. Conventional hydraulic bushings include at least two chambers filled with a fluid. The chambers are connected by a fluid-conducting channel which also has vibration-damping properties. When the hydraulic bearing is subjected to an external force, the volume of one of the chambers decreases to the same degree as the volume in the other chamber increases, with fluid being exchanged through the channel, as mentioned above. In addition, flexible chamber walls can accommodate a change in volume. The chamber walls hereby resist deformation, causing a change in pressure in the chambers. A measure for the pressure change due to volume displacement is referred to as buckling spring rate. The connecting channel equalizes the pressure between the chambers during spring deflection of the hydraulic bushing at low frequencies. Only the elastomeric body is used here as spring support and for damping the support body. An oscillating damped system which consists of the elastic chamber walls and the mass of the fluid residing in the channel becomes increasingly important with increasing frequency. Damping is here produced by the internal friction of the fluid in the channel and/or by its inertia. The damping and hence also the elastic properties change significantly when the hydraulic bearing is excited near a resonance frequency. Above the resonance frequency, the inertia of the mass of fluid in the channel and the friction components prevent additional pressure equalization between the chambers. In this case, the relative stiffness of the chamber walls supports the rigidity of the support and causes an increase in the overall stiffness compared to a low-frequency load. The width, height and position of the resonance can be defined within certain limits by, for example, the flexibility of the flexible chamber walls and by the viscosity of the employed hydraulic fluid. Empirical values for these parameters are known.
Also known are hydraulic bearings which include, in addition to the aforedescribed channel, an additional channel or several channels between the chambers. These additional channels are typically shorter and wider than the main channel and cause an increase in the resonance frequency. The stiffness of the hydraulic bearing is reduced through addition of the at least one additional channel. The driving comfort of motor vehicles equipped with such hydraulic bearings can be significantly improved with these measures. The additional channel must be closed in order to meet the dynamical requirements of the vehicle movements, especially for large load changes, such as acceleration, braking or driving through turns.
It is also known to provide controllable hydraulic bearings with rheological fluids. Such rheological fluids typically change their viscosity and hence their flow characteristics in an applied electric field—for an electro-rheological fluid—or in an applied magnetic field—for a magneto-rheological fluid. When using rheological fluids, the fluidic behavior and therefore the damping characteristic of the hydraulic bearings can be continuously changed and hence also controlled, so that hydraulic bearings of this type are in this context referred to as controllable hydraulic bearings. Controllable hydraulic bearings which permit adaptive control are known, for example, from EP 0 965 006 B1, EP 1 016 806 B1 or EP 0 427 413 A1. A controllable hydraulic bearing based on an electro-rheological fluid is also disclosed in DE 39 10 447 A1. Pulse-width-modulation is here employed which requires rather complex electronics. The use of electro-rheological fluids, however, has its limits where large pressure differences must be supported across relatively large distances between the electrodes.
The inventors have realized that that conventional controllable hydraulic bearings using magneto-rheological fluids disadvantageously operate always with homogeneous magnetic fields. It has been observed that when a pressure limit in the hydraulic bearing is exceeded, the channel which was previously closed by the magneto-rheological fluid, suddenly and abruptly opens. This can result in unwanted noise inside the hydraulic bearing, which is unacceptable in particular when a hydraulic bushing is used to support the chassis of a motor vehicles. It has also been observed that this phenomenon is caused by the homogeneous magnetic field used in conventional hydraulic bushings to date. One possible explanation for this observation may be that the magnetizable particles in the magneto-rheological fluid strongly accumulate at a certain location and then suddenly detach from the walls of the channel when the aforementioned pressure limit is exceeded. This causes the channel to open and allows the fluid to flow from one chamber into the other.
The change in the length of the channel between the chambers, as implemented in several conventional hydraulic bearings, is also disadvantageous for switching processes. In most cases, the channel length which controls the frequencies to be damped inside the hydraulic bearing is altered by employing mechanical solutions. These are relatively sluggish and hence not suited for all applications.